The invention relates to optical signal processing, and in particular an optical device for regenerating optical signals containing phase-encoded data, or quantizing the phase of any input phase modulated signal.
The future of optical fiber communications will be dictated by the need for long reach, high capacity and energy efficient technologies. Transitioning to spectrally efficient modulation formats such as quadrature phase shift keying (QPSK) provides significant capacity gains in long haul optical links. Fully coherent optical signal detection combined with high speed analog-to-digital conversion allows signal processing in the electronic domain, providing capabilities such as compensation for chromatic and polarization mode dispersion, as well as for some of the accumulated nonlinear phase noise which is the dominant limitation in extending coherent transmission spans (see, for example, E. Ip et. al., Opt. Express 16, 753-791; 2008).
However, the power consumption as well as the significant computing overhead associated with the aforementioned electronic functions means (see, for example, K. Roberts et. al., J. Lightwave Technol. 27, 3546-3559; 2009) that a combination of optical signal processing with optical dispersion compensation may still prove competitive for long haul transmission, particularly as signalling rates continue to rise.
A long term goal of research into all-optical signal regeneration has been to identify techniques capable of processing advanced formats with multiple levels in phase or/and amplitude. One proposed scheme utilises a pair of conjugated signal-idler channels transmitted along the link and combined in a non-degenerate PSA (see, for example, M. Vasilyev, Opt. Exp., 13, 7563; 2005); however this wastes valuable transmission bandwidth, and requires impractically low levels of residual dispersion post-compensation. Other proposals to all-optically regenerate QPSK have focussed on scaling schemes intended for use with binary level signals, including an indirect approach utilising format conversion to on-off-keying (OOK), OOK regeneration, and OOK to differential QPSK (DQPSK) conversion (see, for example, M. Matsumoto, Opt. Express 18, 10-24; 2010) as well as a more direct technique using two parallel binary phase shift keying (BPSK) regenerators (see, for example, Z. Zheng et. al., Optics Communications 281, 2755-2759; 2008). Such schemes are significantly complicated by the requirement to fully length-match and stabilise multiple optical paths, as well as a component count that increases appreciably with the density of the modulation format, potentially offsetting some of the economic benefits of the spectrally efficient formats. To date, there have been no experimental demonstrations.